Gas turbine engines typically include several rotor discs which carry a plurality of radially extending aerofoil blades on their periphery. Those aerofoil blades can be in the form of compressor blades or turbine blades. In the case of rotor discs carrying turbine blades, particularly turbine blades for use in the hotter, higher pressure portions of the engine's turbine, it is usually necessary to provide at least some of those turbine blades with internal cooling. Typically, that cooling is provided by cool air tapped from the engine's compressor and directed into passages within the blade interiors. The air is thus placed in intimate heat exchange relationship with the blades, so bringing about a reduction in their operating temperature, before being exhausted into the general hot gas flow through the turbine.
Cooling air is often supplied to the turbine blades through supply passages provided in the rotor disc carrying the turbine blades. Typically, cool air extracted from the compressor is directed along a path close to the engine's longitudinal axis before flowing in radially outward directions and into the disc supply passages. The disc supply passages are generally radially extending so as to direct the cooling air to the radially inward portions of the blades where they communicate with entrances to the cooling passages within the blades.
In use, gas turbine engine rotor discs rotate at very high speeds and are consequently subject to major hoop stresses. The cooling air supply passages within the disc are, of course, subject to the influence of those hoop stresses. Since the passages extend generally normal to the direction of the hoop stresses, there can be a tendency for portions of the passages to act as sites for the initiation and propagation of cracks within the disc. Such cracks are clearly undesirable in view of the limiting effect that they can have upon the life of the rotor disc.
The cross-sectional configuration of the disc passages can be an important factor in the tendency for such cracking to occur. Typically, the passages are of circular cross-section since this is frequently the cheapest and easiest configuration to produce. Under certain circumstances, depending upon the passage diameter and the anticipated rotational speed of the disc, such passages are acceptable. However, there may be other circumstances in which such circular cross-section passages can give rise to cracking.
In U.S. Pat. No. 4,344,738, the problem of rotor disc cracking is tackled by changing the cross-sectional configuration of each disc passage from circular to elliptical cross-section. The major axis of the ellipse extends in a generally circumferential direction so that under the influence of hoop stresses, there is a reduced tendency for cracks to be initiated.